1. Field of the Invention
The present invention generally relates to electronic switches, and more particularly to electronic control of switchable antenna units utilizing a common signal bus.
2. Description of the Prior Art
Antenna arrays are used in a variety of applications, such as underground cable location, radio navigation, and electromagnetic field detection. It is often necessary to route the signals from multiple antennas to a signal processing network in order to prepare the signals for data analysis. Where the application permits, it is desirable to utilize a single signal processing network for the entire antenna array rather than using multiple networks, i.e., one for each antenna. Because of this, systems have been devised which sequentially switch the network input from one antenna to another.
The simplest mechanism for achieving sequential inputs is a manual switch, such as a single pole-single throw contact. A plurality of such devices may be used in an antenna array (one for each antenna), or a multicontact switch may be used. A more convenient alternative is a conventional relay, such as a solenoid coil utilizing an armature contact. An example of a relay used in conjunction with two antennas is disclosed in U.S. Pat. No. 4,387,340 issued to E. Peterman (and in the divisional case, U.S. Pat. No. 4,520,317).
For most applications, however, the switching time of such relays is highly unsatisfactory. An improved, quicker design is illustrated in U.S. Pat. No. 4,542,344 issued to Darilek et al. A simplified version of the electrical configuration of that device is shown in FIG. 1. The prior art structure includes a plurality of loop antennas 1, each connected to one of a plurality of pre-amplifiers 2. The outputs of preamps 2 are connected to a multiplexer 3 which is regulated by a controller 4. Controller 4 selects the antenna or channel to be analyzed at any given time. The selected signal is then directed from multiplexer 3 to the signal processor circuit 5, which conveys data in a comprehensible form to an output device (display) 6 via controller 4.
The foregoing devices still possess a significant disadvantage, to wit, the necessity of providing a pre-amplifier for each antenna. This drawback, which is uneconomical and redundant, is accentuated by the fact that other electrical components may have to be provided for each antenna. For example, FIG. 3 of U.S. Pat. No. 4,295,095 issued to Thomas et al. shows use of a plurality of filters, one for each channel, in addition to the preamplifiers. It would, therefore, be desirable and advantageous to devise means for selectively switching the input of a signal processor from among a plurality of antennas without requiring the redundant use of electrical components for each antenna channel.
Accordingly, the primary object of the present invention is to provide means for selecting a signal from one of a plurality of antennas.
Another object of the invention is to provide such means having a rapid switching time.
Still another object of the invention is to provide switchable antenna units whose output may be directed to a common signal bus.
Yet another object of the invention is to provide a switchable antenna unit that does not require preamplifiers or other signal conditioning components.
A further object of the invention is to provide such a switchable antenna unit having sufficient attenuation of the signal when the unit is not selected.